Subtidal response of the Scotian shelf bottom pressure field to meteorological forcing

Abstract

Data collected during the Canadian Atlantic Storms Program (CASP) show subtidal variations in subsurface pressure (SSP) to be highly coherent throughout the Scotian Shelf region, and well correlated to fluctuations in the alongshelf component of wind stress (τ^y). Analysis using a frequency‐dependent multiple regression model verified that τ^y is the primary source of local forcing to the SSP field, although non‐locally generated variations in SSP are also important. The two components of local wind stress and a non‐local SSP term combine to explain over 90% of SSP variance on the Scotian Shelf.

Statistical results describing the response to τ^y change dramatically depending upon the inclusion of non‐local forcing. In a model including both types of forcing, the SSP response to local forcing behaves like the solution to a dynamical model forced by time‐dependent wind stress with sea‐level prescribed to zero at the eastern cross‐shelf boundary. Local τ^y forcing becomes more effective to the west and onshore, whereas the phase suggests propagation to the west. The importance of τ^y is reduced at higher frequencies. Describing SSP with a statistical model containing local forcing alone may lead to an incorrect interpretation of SSP dynamics, particularly in the synoptic band where the wind variance is greatest.

Energy originating from a non‐local source is most obvious at ω > 0.5 cpd and at locations on the eastern half of the shelf, but plays an important role at all sites and at all frequencies. These variations propagate to the west at speeds of 6.5 (ω < 0.2 cpd), 25–33 (0.2 cpd < ω < 0.5 cpd), and 12–17m s^?1 (ω > 0.5 cpd). The exponential decay scales at all frequencies are ～900 km in the direction of phase propagation. The non‐local response is consistent with theoretical estimates of first‐ and second‐mode shelf waves for this region and represents the most direct evidence of shelf wave activity on the Scotian Shelf to date.